linux-kernel - Re: [PATCH v2 00/39] x86: Kernel IBT

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Message-ID: <20220225172644.76fyxjximjrcdbzt@treble>
Date:   Fri, 25 Feb 2022 09:26:44 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, joao@...rdrivepizza.com, hjl.tools@...il.com,
        andrew.cooper3@...rix.com, linux-kernel@...r.kernel.org,
        ndesaulniers@...gle.com, keescook@...omium.org,
        samitolvanen@...gle.com, mark.rutland@....com,
        alyssa.milburn@...el.com, mbenes@...e.cz, rostedt@...dmis.org,
        mhiramat@...nel.org, alexei.starovoitov@...il.com
Subject: Re: [PATCH v2 00/39] x86: Kernel IBT

On Fri, Feb 25, 2022 at 04:43:20PM +0100, Peter Zijlstra wrote:
> On Fri, Feb 25, 2022 at 04:28:32PM +0100, Peter Zijlstra wrote:
> > +void *skip_endbr(void *addr)
> > +{
> > +	unsigned long size, offset;
> > +
> > +	if (is_endbr(*(unsigned int *)addr) &&
> > +	    kallsyms_lookup_size_offset((unsigned long)addr, &size, &offset) &&
> > +	    !offset)
> > +		addr += 4;
> > +
> > +	return addr;
> > +}
> 
> Damn, I just realized this makes KERNEL_IBT hard depend on KALLSYMS :-(

Why should the jump label patching code even care whether there's an
ENDBR at the jump target?  It should never jump to the beginning of a
function anyway, right?  And objtool presumably doesn't patch out ENDBRs
in the middle of a function.

-- 
Josh